Magnetoresistive effect elements such as a giant magnetoresistive effect (GMR) element and a tunnel magnetoresistive effect (TMR) element having a configuration in which a reference layer as a magnetization fixed layer, a non-magnetic spacer layer, and a magnetization free layer are stacked in this order are known. Among the magnetoresistive effect elements, the TMR element that uses an insulation layer (tunnel barrier layer) as the non-magnetic spacer layer generally has high element resistance but can realize a high magnetoresistive (MR) ratio, compared to the GMR element that uses a conductive layer as the non-magnetic spacer layer. Thus, the TMR element has drawn attention as an element used in a magnetic sensor, a magnetic head, a magnetoresistive random access memory (MRAM), and the like (for example, Patent Literatures 1 and 2 below).
A technology called “spin injection magnetization reversal” in which a spin transfer torque (STT) is applied to the magnetization free layer from electron spins by causing a spin-polarized current to flow through the magnetization free layer is known as a method of reversing the magnetization direction of the magnetization free layer of the TMR element. For example, applying this technology to the MRAM can reduce the size of a memory cell and thus can achieve high density for the reason that an interconnect for magnetic field generation for reversing the magnetization direction of the magnetization free layer is not necessary. Generally, the MRAM that uses the magnetization reversal technology based on the STT is called an “STT-MRAM”.
In the TMR element, the flatness of the magnetic tunnel junction is important. The distance in which a spin-polarized current flows through the tunnel barrier layer is, for example, decreased as the flatness of the tunnel barrier layer included in the magnetic tunnel junction is improved. In the TMR element, such a decrease in distance suppresses a decrease in the degree of spin polarization in the spin-polarized current. Since a voltage is uniformly applied to the tunnel barrier layer more easily as the flatness of the tunnel barrier layer is improved, a current that occurs in the in-plane direction of the tunnel barrier layer is suppressed, and a high MR ratio is achieved. In the TMR element in which the flatness of the tunnel barrier layer is improved, local application of a voltage to the tunnel barrier layer is reduced. Thus, the occurrence of breakdown that damages the tunnel barrier layer is suppressed.